WASHINGTON -- U.S. President Donald Trump’s decision to withdraw from the 2015 Iranian nuclear agreement reverberated throughout the Middle East, Europe, and elsewhere, with experts scrambling to figure out what might come next.

The 2015 agreement, known formally as the Joint Comprehensive Plan of Action (JCPOA), lifted crippling economic sanctions against Iran by the West in exchange for Tehran curtailing -- but not stopping -- its controversial nuclear program.

“This is a horrible, one-sided deal that should have never ever been made. It didn't bring calm, it didn't bring peace, and it never will,” Trump said.

But Trump did not say what comes next in U.S. policy toward Iran, leaving a list of questions that experts are rushing to predict:

Will Washington seek new negotiations with Tehran? Will Iran resume enriching uranium? Will Israel step up attacks on Iran’s proxies, such as Hizballah in Lebanon, or militias in Syria? Will the United States’ European allies try to coax the Trump administration back to the negotiating table with Tehran? Will U.S. forces in Syria become more of a target for Iranian fighters? Will oil prices skyrocket?

Mark Fitzpatrick, former U.S. diplomat and deputy assistant secretary of state for nonproliferation; currently head of the Washington office for the London-based International Institute for Strategic Studies:

“Of all the options that Trump had before him today, he chose the most hard-line position of withdrawing from the Iran nuclear deal altogether. He could have softened the blow. He could have tried to portray some middle ground, suggesting to the Europeans, for example, that there would be several months before any penalties would be imposed so there was still time to try to improve the deal, but he held out no such hope. This is it. He’s pulling out of the Iran nuclear deal and now the ball is in Iran's court.

“This decision today absolutely undermines [the] trust of every country nearly in the world about American leadership, about American commitments to its obligations, about the potential for working with the United States on other areas of mutual interest. Now this is not the first time that President Trump has done something of this nature: pulling out of the Paris climate accords was a huge blow to American leadership and credibility. Pulling out of the Trans-Pacific Partnership [multilateral trade pact] was another such blow. So this is, though, perhaps the most momentous of those blows because it has such a direct impact on the stability of the Middle East, on the potential for an arms race in the Middle East, and on a transatlantic-rift between the United States and its European partners.”

Rachel Brandenburg, former Pentagon adviser involved in U.S. policy toward Iraq and Islamic State, and current director of the Atlantic Council’s Middle East Security Initiative:

“What surprised me more is that he didn't offer any alternatives and he didn't seem to come prepared with a plan for addressing Iran's nuclear program, nor the other threats that he had identified previously…such as Iran’s ballistic-missile program or destabilizing activities in the Middle East. So while he further said this deal is bad and we’re done, he didn't actually offer anything other than further discussions of how to address any of what President Trump acknowledged, and what many acknowledge, is full agreement over the threats of Iran.

“I think there are many, there are many arenas in which [Iran] can respond -- directly or indirectly -- militarily. Right before the announcement we were seeing that Israel was already putting its citizens and forces on high alert in the north, potentially anticipating some sort of response. There’s been some speculation that Iran has kind of been waiting for this announcement before determining how to retaliate against some of the Israeli attacks over the last few weeks. I mean, they could go after U.S. forces in Iraq. They could go through Gaza to hit Israel instead of hitting us directly. There really is any number of ways they could inflict pretty significant pain on the United States or someone as close as partners.”

Robert Einhorn, formеr State Department special adviser during the Obama administration, and a member of the U.S.-Iran negotiating team; currently a scholar at the Washington-based Brookings Institution:

“It was not unexpected and it leaves a lot of details to be announced. Some critical issues that I was interested in, not yet made clear, is he just talking about the waivers due this week or also those in July? Is he, in other words, basically restoring all sanctions that were suspended, all U.S. sanctions that were suspended under the JPCOA? It's not clear yet. Also there's been some reporting to suggest that in addition to restoring the sanctions that were suspended, he would impose some new sanctions. That wasn't clear. He did say that he would impose the highest level of economic sanctions, which suggests that he's really going to get tough with sanctions but we need to see more specifics.

“I think what the Europeans will do will be to try to persuade Iran to stick with the deal. The Europeans will tell the Iranians that even though the United States has essentially left the deal, the deal can remain intact and that Iran can still receive some benefits under the deal and so should stick with it. I think the Iranians will be skeptical that this deal will continue to serve their interest. I think that what President Trump did today will mean that international banks and businesses all over the world will be very reluctant to engage with Iran and many of them that are engaged with Iran will decide to extricate themselves and this will be a substantial economic blow to Iran. I think whether it's sooner or later, Iran will come to the conclusion that this deal is not giving them the benefits that they believe they were entitled to and they will decide to leave as well.

“Russia did not want the United States to withdraw from the deal. Russia thought the deal was working well, contributing to stability, a measure of stability in an unstable Middle East and wanted to preserve the deal. Now that the United States has withdrawn I think Russia will try to take advantage of it. I think it will blame the United States, try to isolate the United States as much as possible.”